# Milky Way - to each according to habitable planet!

01/11/2010

You may have heard reports that more than one quarter of all sun-like stars in the Milky Way may have planets like Earth. The astronomers studied 166 stars within 80 light years from Earth and the planets that revolve around them. They found that about 1.5% of the stars have planets the size of Jupiter, 6% had planets with a mass of Neptune, and about 12% have a planet the size of 3-10 times the mass of Earth.

Although the list is not complete, because the technology does not allow us to detect planets with a mass less than three Earth. But knowing certain statistical information, they can calculate that about 25% of sun-like stars have planets with a mass equal to the mass of the Earth.

How many habitable planets in the Milky Way? Under inhabited refers not only to those whose weight is equal to the mass of the Earth, but are at a distance from their stars, so that water can be in the liquid state (the so-called habitable zone).

You can calculate it. What was done for the sensational world Gliese 581g. The mass of this planet is about three times the mass of Earth, and it is just in the habitable zone. While scientists do not know whether it has atmosphere and what is its composition: yet known about it very little. The implication is that it can be inhabited, was made solely on the basis of information on its mass and temperature.

The solar system Gliese 581 is only 20 light years from us. It’s close enough, compared to the huge size of our galaxy. Let’s assume that Gliese 581g - this is the closest to us is potentially habitable planet. Then we can count the number of potentially habitable planets in the Milky Way! And the calculations are fairly simple.

Let’s extrapolate our example and assume that habitable planets are in our galaxy is 20 light years away from each other (as we shall see, this distance can be much higher or lower, but the result will still impressive). In the picture, you can see the outline of where the stars are located at a distance of 20 light-years apart:

The length of the cube faces is 20 light years away. If one assumes that habitable planets are distributed in the universe that is the case, then one such planet will hold 20 x 20 x 20 = 8000 cubic light years. Assume that the density of habitable planets in the galaxy.

How many cubic light years away in our galaxy?

To simplify the calculations, we represent the galaxy in the form of a right circular cylinder with a base diameter of 100 000 light-years, and the height of the lateral surface of 2,500 light years.

Cylinder volume is calculated as:

V = Pi x radius squared base x height

Consequently, the volume of the universe:

V (the universe) = Pi x (50,000) squared x 2500 = 20 trillion cubic light years!

It remains just divide the volume density of stars in the universe with planets inhabited:

20 trillion. / 8000 = 2.5 billion of potentially habitable planets!

Given the many assumptions and simplifications in our calculations, it should be noted that the actual values can differ by an order of the numbers we received.

Planets 2.5 billion, enough to provide every second person on our own planet Earth. Even if the actual number of habitable planets person is significantly different from that calculated by us, even in this case, in our galaxy, it may be from a few hundreds of millions to hundreds of billions of planets!

As for the method of transportation to these distant planets, it is an engineering problem that will be solved. Maybe in a few hundred years, we will reach a sufficient level of development to overcome it.

Per aspera ad Astra! (Latin: "Through hardship to the stars"), you can always download the news ticker and be aware of all the world’s news.

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